1. Field of the Invention
This invention concerns a method for improving the low temperature properties of an asphalt by heat soaking the asphalt in the presence of a dehydrogenation agent.
2. Discussion of Related Art
Asphalt is a bituminous material resulting from the distillation of crude oil. Typically, asphalt is derived from the bottoms of a vacuum distillation tower and has an atmospheric boiling point of at least 380.degree. C. Because it is hydrophobic and has good adhesiveness and weatherability, asphalt has been used widely as a binder in paving materials and as a coating for roofing shingles.
Shingle coating and some saturants require that the vacuum distilled asphalt be air blown at 200.degree.-300.degree. C. to polymerize the asphalt by the known process of oxidative dehydrogenation in which hydrogen is removed as water vapor in the off-gas. This improves the creep (or flow) resistance and weatherability of the asphalt as well as reduces its sensitivity to temperature changes. Oxidative dehydrogenation can also be effected by using sulfur or sulfuroxygen gases such as sulfur dioxide, chlorine gas, etc., which result in hydrogen sulfide and hydrochloride off-gases instead of water vapor. However, the common practice is to use air blowing.
Conventional paving asphalt binders, by comparison, are not usually air-blown but are vacuum residues which are manufactured to meet certain control specifications such as flash (ASTM D 92), penetration at 25.degree. C. (ASTM D 5), apparent viscosity at 60.degree. C. (ASTM D 2171), and kinematic viscosity at 135.degree. C. (ASTM D 2170). In addition to the control specifications, a paving asphalt should also meet certain performance specifications such as ductility (ASTM D 113), solubility in trichloroethylene (ASTM D 2042), and thin film oven aging (ASTM D 1754).
General refinery practice is to distill crudes deep enough to maximize the recovery of preferred distillate molecules and minimize asphalt pitch production. However, this approach has the disadvantage of producing pitch that is too hard for commercial asphalt application.
This invention overcomes this problem by providing a method to maintain pitch reduction as the refinery objective while concurrently giving the refiner the capability of producing the full range of softer asphalt grades with the added benefit of producing asphalts with improved low temperature performance as measured by an increased penetration and Penetration Index.